1. Field of the Invention
The present invention relates to a method and an apparatus for depositing a film on an object or a good by exposing the object to plasma of a deposition material gas while irradiating the same with ion beams.
2. Description of the Background Art
In a plasma CVD method, an electric power is supplied to a deposition material gas to form plasma from the gas, and a film is deposited on a target object (i.e., an object on which a film is to be deposited) in the plasma. In this method, it has been attempted to perform irradiation with ion beams in addition to the film deposition with the plasma.
For example, Japanese Laid-Open Patent Publication No. 9-208389 has disclosed a method for depositing a film, in which a target object is exposed to plasma formed from a deposition material gas, and the surface of the object is irradiated with ion beams.
For producing the ion beams, an ion beam producing electrode system which is formed of two electrodes has been employed. Alternatively, the electrode system formed of three electrodes may be employed. According to the electrode system formed of three electrodes, the ion beams can be monitored more accurately than the electrode system formed of two electrodes (in the case where the outer electrode is at a ground potential). Further, the former can produce more ion beams and can control more precisely an ion beam divergence angle, compared with the latter.
FIG. 2 shows by way of example an ion source having a beam producing electrode system formed of three electrodes in the prior art. The ion source in FIG. 2 has a cylindrical plasma container 2, which is formed of a radio-frequency (RF) electrode 21 forming a cylindrical sidewall and an RF electrode 22 forming an upper wall. The RF electrodes 21 and 22 are electrically isolated from each other, and an RF power source 4 is connected between the electrodes 21 and 22 with a matching unit 3. A positive power source P1 which can apply a positive voltage to the electrodes 21 and 22 is also connected thereto. A gas supply unit 1 of an ion material gas is connected to the plasma container 2. The gas supply unit 1 includes a mass-flow controller, a valve and a gas source, which are not shown in the figures. In a structure, e.g., including the ion source which is arranged continuously to the film depositing device, the gas introduced into the device and diffusing into the ion source may be used as the ion material gas. In this case, the gas supply unit 1 may be eliminated.
An ion beam producing electrode system E1 formed of three electrodes is arranged in an ion beam producing port 23 of the plasma container 2. The electrode system E1 is formed of first, second and third electrodes E11, E12 and E13, which are arranged in this order from the container 2 side toward the outer side. Each of the electrodes is porous or is provided with slits. The first electrode E11 is electrically continuous to the plasma container 2 which is in turn connected to the positive power source P1. The second electrode E12 is connected to a negative power source P2 for applying a negative voltage thereto. The third electrode E13 carries a ground potential. These electrodes E11, E12 and E13 are electrically isolated from each other. A magnet 100a which provides a magnetic field is arranged outside the plasma container 2 for stably maintaining the plasma.
Positive ion beams are produced from the ion source, for example, in the following manner. An exhaust device, which is employed for the film depositing device provided with the ion source, is operated to reduce the pressure in the plasma container 2 to a predetermined value. Then, the material gas supply unit 1 supplies the material gas of the ion beam into the container 2, and the radio-frequency power source 4 applies a radio-frequency power across the electrodes 21 and 22 via the matching unit 3 so that the plasma is produced from the supplied gas (in the position within the plasma container 2 indicated by "8" in the figure). The electrodes 21 and 22 are also supplied with the positive voltage from the positive power source P1, and thus carry the positive potential so that the first electrode E11 carries the same (i.e., positive) potential as the plasma container 2. The second electrode E12 supplied with the negative voltage from the negative power source P2 carries the negative potential. Thereby, the potential difference between the first and second electrodes E11 and E12 produces the positive ion beams from the plasma 8 formed in the plasma container 2. The third electrode E13 fixes the energy of the ion beams with respect to the ground potential. One of the reasons for which the second electrode E12 is kept at the negative potential is to prevent reverse flow of electrons into the container 2, i.e., ion source, e.g., from the space within the chamber continuous to the ion source and from the chamber wall. The flow of electrons into the plasma 8 in the container 2 is equivalent to the outflow of ions from the plasma 8. Therefore, flow of electrons into the ion source prevents accurate monitoring of the quantity of the produced ion beams.
However, if the first electrode carries a high potential in the ion source having the beam producing electrode system formed of three electrodes shown in FIG. 2, the second electrode is spaced from the first electrode by a long distance for preventing insulation breakage. In this case, if the potential on the first electrode is variable over a wide range, the power source for applying the voltage to the second electrode must have a very wide variable range in order to maintain the substantially constant intensity of the electric field between the first and second electrodes. This increases the size and cost of the power source.
In order to avoid the above, the device may employ an ion source having an ion beam producing electrode system which is formed of four electrodes, e.g., as shown in FIG. 3. This ion source employs an ion beam producing electrode system E2 instead of the ion beam producing electrode system E1 employed in the ion source shown in FIG. 2. The beam producing electrode system E2 is formed of first, second, third and fourth electrodes E21, E22, E23 and E24 arranged in this order from the side nearest to the plasma container to the outer side. The first electrode E21 is electrically continuous to the plasma container 2 connected to the positive power source P1. The second electrode E22 is connected in series to the positive power source P1 and a power source P3, which can apply a negative voltage, so that the negative power source P3 can keep a constant potential difference between the electrodes E21 and E22 even if the output voltage of the positive power source P1 is variable. The third electrode E23 is connected to a negative power source P4 for applying a negative voltage. The fourth electrode E24 carries the ground potential. The electrodes E21, E22, E23 and E24 are electrically insulated from each other. Structures other than the above are similar to those of the device shown in FIG. 2, and the substantially same parts and members bear the same reference numbers.
For producing the ion beams from the ion source shown in FIG. 3, the plasma 8 is formed from the ion material gas in a manner similar to that of the device shown in FIG. 2, and the ion beams are produced from the plasma by the potential difference between the first and second electrodes E21 and E22, and are accelerated by the potential difference between the second and third electrodes E22 and E23. The fourth electrode E24 fixes the energy of the ion beams with respect to the ground potential.
The purpose of keeping the third electrode E23 at the negative potential is to suppress reverse flow of the electrons into the ion source from the position irradiated with the ion beams emitted from the ion source. In this device, since the negative power source P3 keeps the constant potential difference between the first and second electrodes E21 and E22, the power source P3 is required only to apply a relatively small voltage. For example, the positive power source P1 applies a voltage of 10 kV to the first electrode E21, and the negative power source P3 applies a voltage of -500 V to the second electrode E22 so that the potential difference of 500 V is kept between the second and first electrodes E22 and E21, and the positive voltage of 9.5 kV is applied to the second electrode E22. As described above, the electric field intensity for producing the ion beams can be kept constant without increasing the size of the power source P3.
However, the conventional ion sources having the electrode systems which are formed of three or four electrodes suffer from the following problems if the irradiation with ion beams is performed simultaneously with the deposition of the film by the plasma on the target object which is placed in the film depositing device.
In the foregoing ion source having the electrode system which is formed of three or fourth electrodes, and is arranged in the position opposed to the region occupied by the high-density plasma for film deposition, abnormal discharge may occur between the electrodes in the beam producing electrode system. Further, short-circuit disabling the operation may occur. The reason for this can be considered as follows. Positive ions derived from the plasma for film deposition, which have moved into the beam producing electrode system, are located between the first electrode carrying the positive potential and the second electrode carrying the negative potential in the three-electrode system, and is located between the second electrode normally carrying the positive potential and the third electrode carrying the negative potential in the four-electrode system. Particularly, many positive ions are present near the second electrode in the three-electrode system, and are located near the third electrode in the four-electrode system. Therefore, it is difficult to produce the positive ion beams from the ion source due to such positive space charges, and the positive charges filling the space between the electrodes cause the abnormal discharges and others.
In order to avoid the above, the ion source may be arranged in a position remote from the region where the high-density plasma for film deposition is produced, i.e., in a position remote from the region near the electrode supplied with the power for plasma excitation. In this case, however, the ion beams which can arrive at the target object decrease in quantity, and the device increases in size. Such a structure may also be employed that the region, where the high-density plasma for film deposition is produced, and therefore the electrode supplied with the electric power for plasma excitation, as well as the region near the same are shielded against the beam producing electrode system of the ion source in order to avoid direct contact between the beam producing electrode system and the high-density plasma for film deposition. However, this results in complication of the device structure and increase in device size.